A diffractive optical device is an optical device having a grating structure composed of about several hundreds of equally distanced small slits or grooves per micro distance (about 1 mm), and when light enters, a diffracted luminous flux is generated in a direction determined by the pitch (distance) of the slits or grooves and the wavelength of the light. Such a diffractive optical device is used for various optical systems, and one used as a lens that condenses the diffracted lights at a specific degree, for example, is known. If a diffractive optical device is used for an optical system, a chromatic aberration correction effect can be implemented.
As such a diffractive optical device, a multi-layered diffractive optical device as shown in FIG. 12 has recently been proposed. According to the multi-layered diffractive optical device shown in FIG. 12, on a bonding surface of a first diffractive device element 10′ formed of glass and a second diffractive device element 20′ formed of resin, a diffractive optical surface 6′, where a diffraction grating groove formed on the first diffractive device element 10′ and a diffraction grating groove formed on the second diffractive device element 20′ are adhered to each other, is formed. This type of a diffractive optical device in which one diffractive device element and the other diffractive device element are contacted is called a “contact type”. By using the multi-layer diffractive optical device, high diffraction efficiency can be obtained not only at a specific wavelength, but at a wide wavelength range as well.
Besides the above mentioned contact type, the multi-layered diffractive optical device also has a separation type, where two diffractive device elements are disposed facing each other at a distance. In the case of a separation type diffractive optical device, [the two diffractive device elements] having different grating heights, are molded by a die matching the respective grating form. Also in order to satisfy the diffraction condition and obtain high diffraction efficiency, the gratings are aligned. In the case of a contact type diffractive optical device, on the other hand, the grating height is the same and molding is possible with gratings adhered, so the influence of errors of grating heights and grating positions on the diffraction efficiency is small. Hence, in the case of the contact type, a diffractive optical device having a high diffraction efficiency can be easily manufactured.
Mainly glass and resin are used as a material of a diffractive optical device. An example of a diffractive optical device formed of glass and resin is disclosed in the following document (see Patent Document 1). Generally in order to manufacture a diffractive optical device at high volume and at low cost, a die, where diffraction grating grooves are formed, is molded and transferred to glass and resin. In the case of resin, die can more easily be molded than glass. The cost of material is generally lower for resin than glass.
For the contact type multi-layered diffractive optical device, a type where the medium is composed of two layers (see Japanese Patent Application Laid-Open No. 2004-126061), and a type where the medium is composed of three layers (see Japanese Patent Application Laid-Open No. 2003-227912), for example, are known, and in the case of the three layer type diffractive optical device, the optical path difference Δh of the wavelength λ of the light is given byΔh=(n1−n2)h1+(n2−n3)h2,where a refractive index of the medium d1 is n1, a refractive index of the medium d2 is n2, a refractive index of the medium d3 is n3, a grating height of the diffraction grating groove on the bonding surface of the medium d1 and medium d2 is h1, and a grating height of the diffraction grating groove on the bonding surface of the medium d2 and the medium d3 is h2.
If α is a result when the optical path difference is divided by the wavelength λ of the light, α is given as follows,α={(n1−n2)h1+(n2−n3)h2}/λ
If m is a degree of diffraction, the diffraction efficiency η is given as follows,η={sin π(α−m)/(α−m)}2
Compared with the two layer type, the three layer type diffractive optical device can implement higher diffraction efficiency by appropriately selecting the grating heights h1 and h2 of the diffraction grating grooves and the refractive indexes n1, n2 and n3 of the medium. An even higher diffraction efficiency can be implemented by making each medium d1, d2 and d3 of different materials.